In a metal shell of a spark plug, usually, a threaded portion for screwing into a cylinder head is formed in an outer peripheral surface by a threading process. Specifically, a spark plug is attached in a form in which a threaded portion formed on a metal shell of the spark plug is screwed into a cylinder head while clamping a gasket between a gasket seat surface of a gas seal portion formed in the metal shell, and a gasket supporting surface of the cylinder head.
In recent automobile engines and the like, in accordance with tighter regulations on exhaust gas, an air-fuel mixture in the lean (rare) region is often used (a so-called lean burn engine). In a lean burn engine, the fuel mixture ratio is low. Depending on the direction of a ground electrode of a spark plug in a combustion chamber, therefore, a spark discharge gap may be hidden behind the ground electrode from a swirl flow (mixture flow) which is generated during the compression stroke in the combustion chamber, and hence ignition failure sometimes occurs. In such an engine, consequently, it is requested to attach a spark plug with adjusting a ground electrode to an optimum position for ignition. In this case, in order to maintain a regular compression stroke of a gasket, the screw advancement distance in attachment of a threaded portion of a metal shell to a cylinder head is limited to a specified range. In order to adjust the ground electrode to a constant angular position in attachment to an engine, the attachment threaded portion must be rolling-formed on the main shell so that the starting angular position is constant with respect to the joining position of the ground electrode.
In order to satisfy the requirement, Patent Reference 1 discloses a spark plug producing apparatus in which a die and a workpiece are previously positioned so that a constant distance corresponding to the rolling starting position in the direction of the screw thrust is formed between an end face which is opposed to a to-be-threaded portion of a gas seal portion, and an end face of the die corresponding to the positioning end face on the side the workpiece. In this apparatus, while the die and the workpiece are relatively rotated after positioning, the die begins the approach toward the outer peripheral surface of the workpiece so that the thread rolling is started at the thread starting position which is specified in accordance with the joining position of the ground electrode. At the time of the application of Patent Reference 1, however, the production of a vehicle on which such spark plugs are mounted is not largely performed, and the production capacity of such an apparatus is not strictly requested.
Patent Reference 1: JP-A-2001-284015